(a) Field of the Invention
The present invention relates to a recording state detection system for use in a disk drive or data recording/reproducing device and, more particularly, to a reliable data reproducing technique for reproducing data on a recording disk with high reliability and a simple structure.
(b) Description of the Related Art
Intensive studies for developing optical disk drives, such as a digital versatile disk (DVD) drive, and magnetic disk drives are performed for achieving a high-density and large-capacity disk drive. Among other disks, a rewritable optical disk attracts a larger attention due to the advantages of a large capacity and removability thereof.
In an optical disk drive, a semiconductor laser is generally used for recording/reproducing data on the disk, wherein the recorded data is reproduced by focusing a laser beam onto the recording surface of the disk to detect the reflected beam which is modulated by the recorded mark formed on the disk. The detected optical signal is converted into an electric signal, which is fed to a reproducing unit of the disk drive. The reproducing unit includes a waveform equalizer, an automatic gain controller, an encoder or A/D converter etc, and thereby converts an input analog signal into digital data.
The reproduced digital data from the reproducing unit is fed to a PLL (phase lock loop) circuit, which generates a reproducing clock signal in synchrony with the reproduced digital data. The reproducing clock signal and the reproduced digital data are then delivered to a data discriminator for discrimination of data between “1” and “0”, thereby demodulating the reproduced digital data.
In general, data is recorded onto the recording disk by irradiating a recording laser beam, having a higher optical power than the reproducing laser beam, onto the recording surface of the disk to thereby form recorded marks thereon. The optical power of the recording laser beam should be accurately controlled in view of forming a smaller-size mark with accuracy for achieving higher-density recording on the rewritable optical disk. It is difficult, however, to obtain a suitable temperature distribution on the recording surface of the disk in a practical disk drive, even if the optical power is accurately controlled, due to the change of the laser wavelength, distortion of the optical spot, dynamic fluctuation of the ambient temperature and so on.
On the other hand, it is known that the recording disks have variance or scattering in the sensitivity for recording. Thus, upon replacement of the recording disk in the disk drive, a test recording is conducted on a test area of the recording disk before the recording, thereby optimizing the optical power of the disk drive to improve the adaptation between the recording disk and the disk drive. A technique for the test recording is described in JP-A-6-195713, for example.
The technique for obtaining the optimum optical power is described in JP-A-7-153078, wherein a mean difference in the optical power level is detected for a combination pattern having a highest-density pattern and a lowest-density pattern, and the optimum optical power is selected which provides a lowest mean difference (substantially zero difference) therebetween to achieve a symmetry. The user data is recorded onto an optical disk by using the optimum recording optical power, and reproduced therefrom by using a somewhat lower reproducing optical power.
JP-A-9-204665 describes another recording technique using a recording optical power which is obtained by adding a specified value to a recording power wherein the amplitude center of the reproduced signal coincides, or multiplexing the same by a specified value, in view that the recording power which allows the amplitude centers to coincide is not necessarily an optimum recording power which provides a lowest jitter to the reproduced signal.
JP-A-10-55540 describes a technique for obtaining an optimum recording power in a phase change disk, wherein a single single pattern having a plurality of repeated marks which are longer than the optical spot is selected as the test pattern, and the optimum recording power is set at the duty ratio of the reproduced signal therefrom, which is obtained from a mean level and an auto-sliced level of an envelope of the reproduced signal. This is adopted based on the fact that the degree of degradation differs depending on the recorded pattern for the phase change disk. In this technique, it is to be noted that JP-A-7-141659 describes a technique for accurately detecting the asymmetry to determine the optimum recording power.
Several techniques are known for achieving a higher-reliability reproduction of data on the optical disk. In one of these techniques, a direct judgement circuit is used wherein data is encoded by slicing a detected signal. This technique is categorized as a hard decision technique. On the other hand, there are also known soft decision techniques which include a maximum likelihood detection technique, wherein both the preceding and succeeding bits are considered for determining the subject bit of the reproduced data, and a partial response maximum likelihood (PRML) signaling technique which combines a partial response waveform equalization technique and the maximum likelihood detection technique.
The PRML signaling technique detects a maximum likelihood after correcting the reproduced signal by using a waveform equalization technique, thereby adapting the maximum likelihood detector to the specified characteristics of the reproducing channel used therein. For example, a literature entitled “A PRML System on the Optical Video Disk Recorder” in the proceedings of 1994 ITE annual convention, pp287-288 describes the PRML technique.
In an optical disk or a magnetic disk, a high intersymbol interference reduces the reproduced amplitude during reproducing high-density data on the disk. This reduces the SNR (signal-to-noise ratio) for the magnetic disk and the CNR (carrier-to-noise ratio) for the optical disk, both increasing the bit error rate in the reproduced data. The maximum likelihood detection technique detects the data by taking advantage of the characteristics of the reproducing channel having a specified state transition scheme, whereby a minimum RMS (root mean square) of the errors is selected among the RMS of the errors for all the time series patterns conceivable from the characteristics of the reproducing channel. This allows detection of data at a lower bit error rate even with a lower SNR or CNR. In view that this procedure is difficult to use in a practical circuit, an algorithm called Viterbi algorithm, described in “IEEE Transaction on Communication, vol. COM-19, Oct. 1971”, is used for progressively selecting the data path.
The PRML signaling technique will be briefly described hereinafter with reference to FIGS. 1, 2 and 3 while exemplifying a case wherein the most simple channel, PR(1,1) channel, is used for detecting the PRML during reproducing data on an optical disk. The signal reproduced by an optical head is first corrected to generate a PR(1,1) channel by using, for example, an equalizer typically known as a transversal filter. The signal distribution of PR(1,1) channel is exemplified in FIG. 1, wherein the reproduced signals in the channel are distributed on three normal levels Ni, which includes maximum, median and minimum of the normal levels, such as Ni=+1, 0, −1). In this case, the amplitude information Xi encoded at each channel clock changes between two states St=0 and St=1, as shown in FIG. 2, for possible transitions between −1 and 0, 0 and 1, 0 and 0, and 1 and 1.
The most likelihood detection technique is to detect one of the normal level (Ni) series which provides a minimum of square error sum Zn which is defined by the sum of the square of errors Ei=Xi−Ni, as follows:                               z          n                =                              ∑            i            n                    ⁢                                                    (                                                      x                    ⁢                                                                                   ⁢                    i                                    -                                      N                    ⁢                                                                                   ⁢                    i                                                  )                            2                        .                                              (        1        )            
It is difficult, however, to calculate in real time the Ni which provides a minimum for the square error sum zn by calculating for all the possible Ni series. Thus, the Viterbi algorithm is used in this procedure for determining the normal level series Ni. FIG. 3 shows a trellis diagram showing the state transitions of FIG. 2 exploded on a time axis. In the Viterbi algorithm, for each of the two paths input to the state at time instant n, the square error sum zn is calculated from Zn−1 at the time instant n−1 and the value for xn input at time instant n, followed by selecting one of the square error sums zn for the two paths having a lower value. The term “path” as used herein means a graph or route connecting a state and the succeeding state and having a direction therebetween.
The selection of one of the two paths at each time instant in the trellis diagram while back-tracing the paths from the present state toward the past states enables to find a state at which the paths merge (converge). This is called “merge of the paths”. The merging of the paths into a single path means that the reproduced data is fixed, whereby the output corresponding to the path is the result of the detected data. The square error sum zn is generally called a path metric, whereas the path metric at a time instant is called a branch metric.
The Viterbi algorithm is adapted to the state transition of FIG. 2 based on the following progressive formulas:Mn(S0)=min[Mn−1(S0)+(xn+1)2, Mn−1(S1)+x n2];  andMn(S1)=min[Mn−1(S1)+(xn−1)2, Mn−1(S0)+x n2]  (2),wherein Mn(S1) represents the square error sum zn for the case of S1 at the time instant n, min[a,b] represents a function which assumes the value “a” or “b” depending on which value is the minimum, and S0 and S1 represent two states having different values. The path metric is calculated for each of two paths input to each of the states S0 and S1 at the prior time instant, followed by selection of one of the path metrics having a lower value to update the value for the path metric. FIG. 3 shows thick paths having probability at the time instant t9 after the selection of one of the paths at each time instant based on the formulas (2). In this example, the paths are not merged or fixed between t0 and t6, and the paths are merged or fixed between t7 and t9. After the merge of the paths, the paths prior to the merge point are fixed, and thus the outputs qi corresponding to the fixed paths are consecutively delivered for allowing the most likelihood detection.
FIG. 4 shows an example of the Viterbi detector 10 including a branch metric generator 11, ACS (add/compare/select) circuit 12 and a path memory unit 13. FIG. 5 shows the branch metric generator 11, which receives input signal yi to generate specific signals (yi+1)2, yi2 and (yi−1)2. These specific signals are fed to the ACS circuit 12, which adds these specific signals to the path metric signals Mn(S0) and Mn(S1), compares the added data against each other, selects one of the added data for updating the current path metric signals. By iterating these procedures, the paths are merged, thereby allowing detection of the maximum likelihood. In addition, the result of the comparison representing the path selection information is stored in the path memory unit 13 and then output as the bit information corresponding to the fixed paths before the merge, whereby the data detection is performed.
In the technique described in JP-A-7-153078, the recording (optical) power, determined by a specified test pattern recorded in a specified area for adjusting the optimum recording power, necessitates recording/reproducing the specified test pattern after moving the optical head to the specified area. In addition, an analog circuit is used as the asymmetric detection circuit for detecting the specified pattern. Further, the shortest signal for use in the asymmetry detection has a smaller CNR after high-density recording, which degrades the detection accuracy.
In the technique described in JP-A-9-204665, in view that the recording optical power wherein the amplitude centers of the reproduced signal coincide with each other is not necessarily such that provides a minimum jitter for the reproduced signal, the technique uses a recording power obtained by adding or multiplying a specified value to/by the thus obtained recording optical power. However, in the Viterbi detector, it is important to form a recording mark adapted to the characteristics of the reproducing channel, and a recording technique without the symmetry does not necessarily provide an optimum recording. In addition, since there are a plurality of combinations for the disk drive and the disk, the constants provided for respective combinations of the disks are difficult to determine.